1. Field of the Invention
The present invention relates to a compact scanning beam antenna and, more particularly, to a compact scanning beam antenna feed arrangement which comprises a linear phased array of small feed elements forming an approximate line source and a subreflector which is shaped to focus an approximate wedge-shaped cylindrical beam generated by the line-source linear phased array to a point source to produce a sherical wavefront which can be reflected by a main parabolic reflector to convert the spherical wavefront from the point source into a planar wavefront aimed toward a predetermined direction at the aperture of the antenna.
2. Description of the Prior Art
Recently suggested designs for future generation satellite communication systems have proposed the use of one or more scanning spot beams at a satellite switching repeater for separately receiving and transmitting signals associated with a plurality of remote, spaced-apart, ground stations. One of the most recent designs, also forming the subject matter of a copending patent application Ser. No. 33,735, filed for A. Acampora et al on Apr. 26, 1979, now U.S. Pat. No. 4,315,262, and assigned to the same assignee, incorporates a satellite switching repeater which uses a plurality of linear scanning spot beams to concurrently scan along separate parallel strips of the overall ground service region of the satellite communication system in accordance with a predetermined communication sequence. In the arrangement of the copending application, each set of a linear array of feedhorns is located in the focal plane of a cylindrical parabolic reflector oriented parallel to the linear array feeds. Each row of feedhorns acts essentially as a line source radiating a wavefront which is transformed by the reflector into a spot beam in the far field of the cylindrical reflector.
Another linear scanning antenna is disclosed in U.S. Pat. No. 4,250,508 issued to C. Dragone on Feb. 10, 1981 which relates to a feed arrangement comprising a linear array of feed elements disposed within a rectangular waveguide section including an offset curved focusing reflecting surface which bidirectionally converts an essentially planar wavefront from the array into a converging wavefront that is focused to a focal point on the focal plane of the feed arrangement. When the array is scanned in one angular coordinate the resulting point sources at the focal plane move along a straight line and are always directed at a predetermined remote point beyond the focal plane regardless of the direction of scan along the one angular coordinate.
Still another linear scanning type antenna arrangement is disclosed in U.S. Pat. No. 4,259,674 issued to C. Dragone et al on Mar. 31, 1981 which relates to an exemplary Gregorian phased array antenna arrangement. There, a main parabolic reflector and a parabolic subreflector are arranged confocally so that a magnified image of a linear feed array disposed along an array plane is formed over the aperture of the main reflector. Also included in the antenna arrangement is a filtering means to reduce grating lobes, and more particularly, for placing a filter at one of the antenna arrangement's real focal points in such a manner as to block the grating lobes due to the array structure while allowing the central ray to pass through the filter. The linear phased array of such arrangement is shown as comprising, for example, a plurality of long feedhorns for also minimizing the phase error in the plane wave feed arrangement.
Since both weight and volume are severely restricted in a satellite, the problem remaining in the prior art is to provide a phased array antenna arrangement and feed which is more compact than prior art arrangements and still provides comparable performance characteristics.